Muscle and Electric Organs 605 



muscle. Here in Pecten is a combination of mechanical "catch" mechanism 

 and effectiveness of occasional motor impulses. The "catch" mechanism is 

 turned off by an inhibitory volley. 



Muscles differ greatly in their ability to maintain tension. These differences 

 are due in part to their mechanical properties and in part to the nature of 

 their nervous and hormonal control. Measurements of mechanical properties 

 have been made under such diverse conditions that it is impossible at present 

 to arrange muscles in any logical series even though such an arrangement 

 would be useful. The vague term "viscous-elastic properties" must be re- 

 placed by quantitative data regarding the mechanical properties of connec- 

 tive tissue and of contractile proteins. A comparative study of the mechanics 

 of muscle might be a most valuable approach to the basic problems of con- 

 traction and relaxation. 



RHYTHMICITY AND THE EFFECTS OF DRUGS 



Not only do muscles differ in their rates of contraction and relaxation, 

 but some muscles are capable of maintaining rhythmic activity. In some 

 hearts the rhythm originates in muscular tissue, whereas in others there are 

 nervous pacemakers (Ch. 15). The muscles of neurogenic hearts may beat 

 in embryos before the pacemaker ganglia develop. The origin of spontaneity 

 in rhythmic muscles is sometimes in the muscle cells and sometimes in con- 

 tained nerve elements; for skeletal muscles the rhythm usually originates in 

 the central nervous system. 



Rhythmic movements such as breathing and locomotor reflexes require 

 tetanic trains of nerve impulses from specific nerve centers. Under certain 

 abnormal conditions, as when immersed in pure NaCl solution or in cal- 

 cium-precipitating agents such as citrate, striated muscle twitches spontane- 

 ously. A deficiency of calcium causes a tendency toward increased excita- 

 bility and slowed accommodation (delayed rise in threshold during stimu- 

 lation). Tetany in parathyroid lack is accounted for on this basis. 



The body wall of many tubular animals shows spontaneous rhythmic move- 

 ment. In the earthworm the normal movements of crawling are reflex in 

 origin (Ch. 23). Spontaneous contractions have been seen, however, in 

 strips of earthworm body wall lacking ventral nerve cord. 



In holothurians (Thyone) the cloacal muscles show periods of rhythmic 

 activity initiated from the nervous system. *''^^ Similarly in coelenterates gen- 

 eral body rhythms are responses to rhythmic nerve centers, such as the 

 "sense organs" of medusae. 



Muscles of the visceral type show spontaneous contractions which may 

 occur in the absence of nerve cells. In embryos the amnionic muscles, like 

 the heart muscle, contract rhythmically without innervation. The stomach 

 and intestinal wall of mammals contain inner circular and outer longitudinal 

 muscle fibers; between them is the myenteric plexus of Auerbach which 

 is connected to the plexus of Meissner in the submucosa. Local rhythmic 

 contractions, segmental and pendular movements, occur in intestinal muscle 

 when the nerve plexuses have been inactivated by nicotine or cocaine''^' ^^^ 

 and in circular muscle stripped from the plexuses.^ ^"' Part of the electrical 

 activity of intestinal muscle is associated with contraction, but some action 



